Feed Validator – By entering your feed URL at Feed Validator, the service will literally translate your RSS to code and identify any potential problems. It works with versions of RSS up to 2.0.
RSS 1.0 Validator – This validator for RSS 1.0 generates an HTML page identifying any errors.
Redland RSS 1.0 Validator & Viewer – This validator offers a variety of display options for checking your feeds.
The W3C Feed Validation Service is a free service that checks the syntax of Atom or RSS feeds. The Markup Validation Service is also available if you wish to validate regular Web pages.